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Speed made good over the watch: the ground, not the log
The rule
- Measure the distance on the chart between the fix at the start and the fix at the end of the watch: that is where she actually got to.
- Divide it by the whole length of the watch. A stoppage stays in, because the next watch will have its own delays.
- The log measures water past the hull, not ground covered; keep its distance for the comparison, not for the speed.
- Log distance − ground distance shows the current along her track (and any log error): note it every watch.
Formula
- Speed made good = distance between the plotted positions ÷ whole length of the watch
- Current along the track (and any log error) = log distance − distance made good
Worked example
| Distance made good between the two plotted positions | 53.6 nm |
| Distance shown by the log over the same watch | 56.2 nm |
| Length of the watch | 4 h |
| Time the engines were stopped during the watch | 0.5 h |
- Ground covered = distance between the plotted positions = 53.6 nm (not the log's 56.2 nm)
- Time = the whole watch, stoppage included = 4 h
- For the record: log − ground = 56.2 − 53.6 = 2.6 nm, the current (and any log error)
- Speed made good = 53.6 ÷ 4 = 13.4 kn
Answer13.4 kn
Common mistake
Dividing by 3.5 h, the hours she was moving, instead of the 4 h watch.
15.31 kn is what she did between stops; an ETA built on it assumes the rest of the passage will have no stops at all, and she arrives late.